A conventional CMOS operational amplifier (op amp) may comprise a transconductance stage and an output driver stage. In a first order approximation, such an op amp is a two-pole system; one dominant pole from a Miller capacitor C.sub.M at the output Of the differential transconductance stage g.sub.M and a second non-dominant pole associated with the output capacitive load C.sub.L at the drain of transistor Q as shown in FIG. 1. The dominant pole stabilizes the op amp by decreasing the loop gain below unity, ideally before the non-dominant pole becomes effective, thereby avoiding oscillations. Since the transconductance stage g.sub.M typically has high gain and a high output impedance, the Miller capacitor C.sub.M is made small in high frequency applications to increase the open loop bandwidth of the op amp, while the output stage transistor Q is made large to drive the external capacitive load C.sub.L.
To increase the bandwidth and operating speed of the conventional op amp, the second non-dominant pole (1/R.sub.Q C.sub.L) can be moved to a higher frequency which is usually done by increasing the transconductance of the output device, or by inserting gain (like amplifier A) between the transconductance stage g.sub.M and the output stage transistor Q. The insertion of amplifier A creates a minor loop in the op amp, through output stage transistor Q, Miller capacitor C.sub.M and amplifier A, with a dominant pole from capacitor C.sub.L and the output impedance of transistor Q (1/R.sub.Q C.sub.L), and a non dominant pole associated with amplifier A and the gate capacitance of transistor Q (1/R.sub.A C.sub.Q). In order to keep the minor loop stable, the frequency of the non-dominant pole has to be much higher than the unity gain frequency of the minor loop .omega..sub.u =g.sub.MQ .times.A/C.sub.L, say by a factor of four. Unfortunately, the introduction of amplifier A moves the unity gain frequency .omega..sub.u closer to the non-dominant pole frequency of the minor loop and increases its output impedance which moves the same non-dominant pole closer to the unity gain frequency .omega..sub.u. The combination contributes to instability of the minor loop at higher operating frequencies thereby limiting the bandwidth of the op amp. Thus, there is a trade-off between (a) using amplifier A to increase the non-dominant pole and unity gain frequency of the entire op amp for more bandwidth, and (b) de-stabilizing the minor loop with to much gain via amplifier A.
A fully differential version of the aforedescribed op amp is shown in FIG. 2 with amplifiers A1 and A2 driving separate output stage transistors Q1 and Q2. The compromise between bandwidth of the op amp and the stability of the minor loop also applies to the fully differential prior art, wherein the amplifiers Al and A2 increase the non-dominant poles of the op amp for a more bandwidth although at the expense of introducing instability in the minor loops.
Hence, what is needed is an improved operational amplifier having gain inserted before the output stage transistor for more bandwidth without causing stability problems.